1. Field of the Invention
This invention relates generally to systems for feeding or delivering rivets from supply bowls or the like to the installing heads of riveting machines, and more particularly to automatic systems for delivering rivets of various sizes and types for installation in workpieces having correspondingly various thicknesses and other parameters.
2. Prior Art
(a) Generally--Prior art in this field is rather limited, because automatic rivet selection has only become of interest in recent years. Two factors account for the recent attention to automatic selection: tighter tolerances, and the availability of automatic measurements.
The requirement for closer tolerances arises from the recent development of more-sophisticated fasteners (some, for example, with spin-on nuts), whose length must more closely match the workpiece thickness. Automatic measurements are the result of the computerization of industry in general, though it must be noted that in the riveting industry--before the improved measurement techniques of the present invention--automatic measurements have not been fully satisfactory.
In any case, earlier automatic rivet-feeding systems have been plagued by unreliability, or by the need for manual performance of certain parts of the delivery sequence, or by both.
Some attempts to overcome these aggravations have identified pneumatic delivery as a source of unreliability, and accordingly have eliminated pneumatic delivery and have focused on minimization of the delivery-path length. One key result--as explained below--has been a clutter of feed-system parts in the immediate vicinity of the installing head of the riveting machine.
Such misallocation of the work space, in turn, produces severe inaccessibility of both the installing head and the workpiece, as well as very considerable inconvenience in servicing and resupply of the feeding system itself. (Typically, access to the feeding system requires the technician to clamber over the workpiece, incurring the additional risk of damaging it with dropped tools or equipment.) With some workpieces having strongly contoured shapes--such as aircraft nacelles--that curve upward from the riveting site, the clutter of feed-system parts around the riveting head can be simply prohibitive. That is to say, such feed-system parts can positively obstruct the workpiece, and thus render the system unusable for such workpieces.
Another pervasive feature of prior systems has been the practice of taking measurements of workpiece thickness at a hole adjacent to the one in which the corresponding rivet will be installed. There has never been any real physical reason for this practice as applied to automatic feeding systems, but it has grown up as standard practice in the riveting industry.
More specifically, this practice originated before automatic feeding systems came into use, when changing rivet sizes was a relatively time-consuming matter. In that context, it appeared reasonable to deliver a rivet in advance, via an injector to the gripping fingers of the riveting machine, so that once the rivet hole was drilled the rivet could be installed and upset without further delay. By using advance delivery, designers made it unnecessary to wait for measurement, rivet-size selection, and feeding-system manipulation, before the rivet could be driven home. The most practical way in which this could be accomplished was to use each measurement to select a rivet for use in the next succeeding hole.
This latter practice, of course, depends heavily on the wishful premise that the workpiece thickness does not vary abruptly. In some commonplace industrial applications, there are occasional transverse pieces to be riveted to the main workpiece--such as the stabilizing elements ("stringers") at intervals along aircraft wings. Such cases of course entail gross departures from the premise that there are no abrupt thickness variations. Dealing with such gross departures, when using an adjacent-measurement system, requires woefully elaborate programming or other provisions.
The measurement of workpiece thickness (and/or other parameters) has been subject also to its own problems, in addition to or perhaps merely aggravated by the excessive crowding of components at the riveter head. Such few automatic measurement systems as have appeared commercially have been extremely inconvenient in use, primarily by virtue of having fixed "reference" or "tooling zero" positions.
In such measurement systems the nominal "zero-thickness" position of the probe that measures workpiece thickness is fixed in relation to stationary components (such as the riveter "C"-frame and housing). This fixed relationship is at odds with the variability of tooling for various jobs, including tooling components both above the workpiece (such as the "pressure-foot bushing" used in most riveters) and below.
Consequently a fixed-reference system renders a routine tool change, or even a tool adjustment, a major operation. Such routine procedures, with a fixed-reference system, require readjustment of the entire measurement sensor--and access to that device is often very awkward.
Even with all these adverse side-effects, however, the primary problems of reliability and/or of necessity for manual operations have remained.
(b) Tube systems--The difficulties with pneumatic delivery, and with both tube-storage and tube-delivery systems generally, can be classified into two types--jamming of multiple rivets in a tube, and jamming at intersection points. As to the first of these, it has been established rather definitely that accumulation of multiple rivets above a release pin or gate produces almost invariably a rivet-jam of one kind or another. This fact has given tube systems a bad name, which is quite justified with respect to tube storage of rivets. Its application, however, to tube transport of rivets has been somewhat unreasoning, since this type of jam is avoidable in systems transporting only one rivet at a time.
Jamming at intersection points is another matter. Such intersection points include intersections between a tube and the device at either end, and also points of convergence between two or more tubes as in a tube-type manifold. Prior attempts to use tube transport systems have almost all faltered over this group of problems, and particularly over the tendency of rivets to tumble into unanticipated transverse (or inverted) orientations when dropped or blown into or out of a tube, or when traversing an area of convergence between two tubes.
Frustration with these pervasive problems has led to some tendency for a teaching, in the prior art, away from tube transport of rivets and toward track transport.
In particular, efforts to provide reliable "escapements" (devices for moving rivets from storage containers called "feeder bowls" into supply paths) in pneumatic-transport systems have been unsatisfactory. For example, conventional escapements rely on a single-blade shuttle to contact each rivet by its shank and to carry it to the entry point of a supply tube. These escapements fail to establish reliable orientation control.
In one relatively advanced system the escapement proper ejected rivets onto a short section of track; the rivets slid on the track to a tube, into which they were "vacuumed" by a Venturi construction. The air-flow adjustment, even with this elaborate arrangement, was very critical, and the entire device accordingly was very "fussy." Taking the temperamental and somewhat unstable character of the air adjustment into consideration, even this advanced system must be regarded as relatively unreliable.
(c) Track systems--The use of track transport, however, has been fraught with its own drawbacks. Rivets are lightweight and slide along such tracks under the influence of gravity, so there are inherent tradeoffs between delivery time, physical length of the delivery path, the height of the supply bowls and manifold above the riveter, and the need to keep the tracks clean, free of corrosion, and unkinked.
The solution to this multidimensional problem has most popularly taken the form of short, very steep tracks--requiring all the equipment to be clustered rather closely about the riveter head, with the severe disadvantages already noted.
Other disadvantages of track systems include inflexibility, with respect to rearrangement of equipment when workpiece types are changed. Running successive jobs that require substantially different combinations of rivet types and sizes therefore entails major delay and expense, as skilled personnel rearrange and modify the tracks and the associated devices.
Track systems, furthermore, generally make use of an upper or retainer track to keep rivet heads from leaving the guide tracks. The spacing between the retainer track and the guide tracks must be relatively close, if the retainer track is to do its job, but must not be too close, lest rivet heads bind between the two tracks at very minor kinks in the track structure. This set of interrelated constraints makes track systems very temperamental, since minor kinks can be almost unnoticeable except by binding of rivets in them; and also makes track systems relatively inflexible as to the capability of transporting rivets of different head sizes.
The transverse space between the guide tracks is similarly "fussy," resulting in inflexibility of track systems as to the capability of transporting rivets of different diameters. Such inflexibility may of course be cured by using track structures with adjustable separations between the guide tracks, but this adds to the expense and to all the other difficulties already outlined.
Yet even this severely adverse set of compromises has not sufficed to resolve the problems of convergence from several supply bowls, containing rivets of different sizes, to a common delivery path. Tracks, like tubes, have open spaces at points of convergence, and these open spaces are virtual invitations to tumbling or twisting--and, consequently, jamming.
As a result, some commercial systems actually call for the equipment operator to shift the "upstream" end of a delivery track from one supply track to another manually, as the workpiece requires rivet-size changes. In some cases the operator must actually remove the workpiece to effect a rivet-size change. Others may call for the operator to operate a control which effects this repositioning by motor or solenoid, but the adequacy of this approach is questionable in view of the temperamental character of tracks at junction points.
Most or all conventional escapement mechanisms only contact the bodies of the rivets, and often allow rivets to fall--sometimes inverted--by sideward motion into the track.
(d) Transfer stations--Some recently innovative systems have introduced specialized apparatus designed to achieve positive control of rivet orientation at the junction point of several supply paths with a common delivery path to the riveting machine--that is to say, at the point where any one of various supply paths from various storage bowls must be selectively extendable to an injector. The apparatus introduced for this purpose hands off one rivet of the required size from the appropriate supply path to the delivery path. Such a device may be called a "transfer station."
One prior-art transfer station, reportedly introduced by the Gemcor corporation, consists of a unitary long block with a hole drilled all the way through it in the long dimension, and a number of small turntables--with rotational axes perpendicular to that of the long hole--arrayed along the hole and intersecting it. The number of turntables is rather high, sixteen being apparently a customary number.
Each turntable has a pair of holes drilled through it along diameters. These holes intersect each other in the center of the turntable. When a particular turntable is in place in the block the turntable is rotatable to bring either of the diametral holes into alignment with the through-hole in the block.
In addition, the block has a number of lateral port holes through which rivets may be introduced, and these port holes are positioned so that their axes intersect the lengthwise hole through the block at the turntable centers. The angle between each port-hole axis and the lengthwise-hole axis is--after the system has been aligned--equal to the angle between the two diametral holes in the corresponding turntable. By virtue of this equality, the turntable is positionable so that one of the diametral holes is aligned with the lengthwise hole in the block and the other of the diametral holes is aligned with the corresponding lateral port hole. These two holes will be called the "first diametral hole" and "second diametral hole" respectively, in the rest of this discussion.
When all the turntables are aligned in just this way, the second diametral hole of each turntable can receive a rivet from a supply tube, so that there is a rivet waiting in each turntable. By virtue of a stationary split stop pin placed near the center of each turntable, however, the rivets do not extend into the lengthwise hole in the block; thus the first diametral holes of all the turntables are unobstructed, and form with the lengthwise hole in the block a straight transfer path from one end of the block to the other.
When a particular turntable is rotated so that its second diametral hole is aligned with the lengthwise hole in the block, the rivet in that turntable is able to bypass the split stop pin and pass out of the turntable into the lengthwise hole in the block. Transport air is introduced at one end of the block to facilitate this passage. The rivet is blown through the lengthwise hole in the block to a delivery path--that is, to a path that leads to an injector adjacent to the riveting machine. This path is at the end of the block opposite that at which the air is introduced.
Rivets in the turntable that is nearest the delivery-path end of the block must pass only through a short section of the lengthwise hole in the block to reach the delivery path. Successful transfer of such a rivet therefore depends upon the accuracy of alignment of the first diametral hole in the turntable with the lengthwise hole in the block. This alignment accordingly must be adjusted carefully when the system is set up, and the adjustment maintained during operation.
In addition, the lateral input port in the block should be made adjustable to align with the second diametral hole in the turntable. (Alternatively, extremely high-precision machining of the block and all the turntables could be used, to eliminate the necessity for adjustability of the lateral input port holes. The cost of this approach, however, would be more onerous than the requirement of providing and using adjustable ports.)
Thus this particular turntables can be used to successfully deliver to rivet of some one sizes, relying only upon two adjustments--generally a satisfactory state of affairs. For all other turntables, however, a discharged rivet must pass through all the other downstream turntables, so that the rivet at the transport-air end (or "upstream end") of the lengthwise hole must negotiate all sixteen of the first diametral holes to reach the delivery end. Delivering a rivet from the upstream turntable consequently depends upon the alignment adjustment of all sixteen of the first diametral holes, plus the alignment adjustment of the lateral port hole to the second diametral hole of the upstream turntable: seventeen independent adjustments in all.
In effect, the adjustments become interdependent--there is an interaction between alignments for different rivet sizes--since all must be correct to deliver even one rivet.
This system is undoubtedly usable, and doubtless performs a useful function, but the reliance upon multiple alignments for delivery of only a single rivet (except for the rivets in the furthest downstream turntable) makes the system either inordinately costly or extremely tedious to adjust and extremely temperamental. In this particular transfer-station design, it may be helpful to conceptualize these disadvantages as associated with the fact that the rivet delivery trajectory is a compound motion: (1) a rotary motion to line up the second diametral hole with the lengthwise hole in the block, then (2) a linear motion of the rivet itself through the first diametral hole of each other downstream turntable, to reach the delivery path at the end of the block.
Another perspective is that these disadvantages result from trying to transfer all the different rivet sizes from a common transfer station. As more and more rivet sizes are desired, for a particular complexity of workpiece and thus riveting procedure, the transfer station becomes longer and longer. Presumably, in a system of this type designed to handle sixty-four different rivet sizes, delivering just one rivet of just one size from the upstream turntables would require perfectly adjusting as many as sixty-five different alignment stops.
Another prior-art transfer station, attributed to a German manufacturer, reportedly makes use of a carousel to receive a large number of different rivet sizes--each in a separately movable transfer shuttle carried on the carousel. To deliver a rivet of any one of these sizes, first the carousel rotates to line up the corresponding shuttle with an actuator and/or a track, and then the shuttle is actuated forwardly along the track to the center of the carousel. When the shuttle reaches the center of the carousel, the rivet is dropped and/or blown out of the shuttle into a delivery path.
In this system successful transfer of any one rivet size requires accurate adjustment of the receiving position of each shuttle to align with the supply path from its corresponding rivet feeder bowl--or requires continuously attached flexible supply paths, whose downstream ends rotate with the carousel. The latter design choice poses problems of its own. In addition, successful transfer requires accurate adjustment of the delivery position of each shuttle to align with the common central delivery path. Unfortunately, these two adjustments are not the only ones required to transfer a rivet, since the carousel too must be made to line up the shuttle of interest with the actuator and/or track.
In principle the system could be provided with a guide that accepts the shuttle over a relatively wide range of positions, and funnels it into a progressively "tighter" trajectory to the center of the carousel. In practice, however, this type of guiding arrangement would pose operational problems of its own. Consequently the rotary stops of the carousel must be made quite precise. Rotary stops, however, by their intrinsic geometry cannot be configured as positive limit stops; they must be detents or the like.
Detents are notoriously subject to wear, inherent imprecision, and unreliability. They are also likely to be very fussy to adjust. Consequently this transfer-station design is inherently flawed by virtue of its dependence upon detents and/or a temperamental guide system.
In this system as well as the system previously discussed, it appears helpful to associate the system drawbacks with a compound transfer motion. The transfer motion in this system, in fact, is even more pronouncedly compound than that in the Gemcor system described above. In the carousel system, transfer of one rivet requires (1) a rotary motion to align a particular shuttle with the actuator/track, and (2) a rectilinear motion to move the shuttle into the center of the carousel. Considering only the transfer sequence, therefore, the faults in this type of system are clearly associated with the use of a compound transfer motion or transfer trajectory--as distinguished from a single-stage motion.
Furthermore, unless a flexible-tube supply system is used, the carousel must (3) rotate back to a resupply position to replace a rivet in the emptied shuttle, before another rivet of the same size can be transferred. Since it is commonplace to require transfer of several rivets in a row that are the same size, this last feature introduces a good deal of extra motion, leading to wear and breakdown.
The three motions required to transfer and resupply any one rivet size can also absorb significant amounts of extra time.
The use of compound transfer motions, at least in the two prior systems of which we have heard, appear to be related to the various drawbacks of both systems; it may be concluded that such motions are highly undesirable. It is also fair to draw the generalization that attempts to transfer rivets of a large number of different sizes give rise to transfer stations having a large number of individually actuable transfer devices, and that having a large number of individually actuable transfer devices tends to require compound transfer motions.
It may be objected that this line of reasoning appears to lead to limitation of the number of rivet sizes which can be transferred. In fact that is not so; this reasoning only leads to limitation of the number of sizes transferable in a single transfer station. We have found that the use of a single station to transfer a large number of rivet sizes is undesirable, and that discarding the requirement of using a single station not only eliminates compound motions and their deficiencies, but also introduce certain other important benefits.
(e) Automatic measurement--Prior commercial measuring systems have used linear potentiometers, mounted to generate singals related to measuring-probe position. Such potentiometers provide, in effect, a fixed reference zero.
In use such a potentiometer may be adjusted so that its reference-zero position occurs at the point where the probe bottoms out--that is, where the probe position corresponds to zero workpiece thickness. If the workpiece is rather thick, however, this type of adjustment produces rather small gradations in output signal (that is, small differences relative to the total signal) for significant gradations in thickness.
In particular, the differences between signals require to reflect functionally different rivet lengths may be very small. This type of system therefore severely tests the linearity of the potentiometer, and requires the monitoring electronics to respond precisely to small differences superimposed on relatively large signals--always an unfavorable operating condition, for any measuring system.
An alternative is to adjust the potentiometer so that its reference-zero position occurs at some higher probe position, perhaps corresponding to the average anticipated workpiece thickness or some value near to that. This alternative, unfortunately, requires the use of numerous different gauge blocks or otherwise calibrated offsets--one such offset close to each range of thicknesses to be encountered in operation. The result is to introduce yet another elaboration and complexity into a system whose operation should be as efficient and convenient as possible.
Yet these systems, to the extent of their application as discussed so far, while aggravating are not prohibitively inconvenient to operate. That very condition, however, arguably sets in when one considers the difficulty of making tool changes or even tool adjustments. Such procedures should be routine in almost any industrial riveting operation, but as already mentioned they require recalibration of the measuring system.
Such recalibration, using a linear potentiometer, entails manually shifting the entire potentiometer to reposition its zero point. This is an expensive annoyance in any case, but particularly when the riveter head is inaccessible or when the zero must be found using several separate gauge blocks or other offset calibration.
Using a helical potentiometer or "helipot" for these applications might be helpful in reducing the sensititivity to potentiometer nonlinearities, but the linearity problem in the electronics would persist.
Furthermore, even with the best of prior-art measurement systems, the range of workpiece thicknesses over which measurements may be taken is severely limited--specificially, it is limited to the length of longitudinal travel of the potentiometer wiper (whether linear or helical).
As already mentioned, one of the most significant limitations in prior-art measuring systems has been the custom of taking the measurement at some other location than the one in which a rivet is about to be installed. In some cases this causes substantial inaccuracy in the measurement, and in some systems the measurement at each point is actually made after installing a rivet there, so that the only possible corrective action is a fully manual replacement procedure later.
All of these inconveniences, costs, and delays of the prior-art rivet feeding systems are eliminated by our invention, which nevertheless is relatively simple and economical to construct and to use.